Photolithography or optical lithography systems used in the manufacture of integrated circuits have been around for some time. Such systems have proven extremely effective in the precise manufacturing and formation of very small details in the product. In some photolithography systems, a circuit image is written on a substrate by transferring a pattern via a light or radiation beam (e.g., UV or ultraviolet light). For example, the lithography system may include a light or radiation source that projects a circuit image through a reticle and onto a silicon wafer coated with a material sensitive to irradiation, e.g., photoresist. The exposed photoresist typically forms a pattern that after development masks the layers of the wafer during subsequent processing steps, as for example deposition and/or etching.
Due to the large scale of circuit integration and the decreasing size of semiconductor devices, the reticles and fabricated devices have become increasingly sensitive to overlay errors, as well as other critical parameter variations such as critical dimension variations, etc. These variations, if uncorrected, can cause the final device to fail to meet the desired performance due to electrical timing errors. Even worse, these errors can cause final devices to malfunction and adversely affect yield.
Improved apparatus and techniques for determining overlay error, and the like, on a semiconductor target are desired.